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Topic Title: A head scratcher!
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Created On: 24 July 2014 06:11 PM
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 25 July 2014 07:45 AM
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leckie

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I woke up early and the first thing I thought of was this flipping fault! I hate it when I can't work something out

I have had a good think about this and done loads of drawings but still cannot work out a scenario that would cause 12.5A in the line and the neutral and the 25A reading across the L and N without two 3kW loads being connected. Each immersion is supplied from separate boards. The off peak board mcb has only the one cable connected, so it's not supplying two heaters. The on peak immersion is connected via a SW fuse spur which is switched off. So even if there was a cross connection between the circuits where does the addional 12.5A come from?

I tested for voltage on the primary side of the off peak main switch. I did wonder if the off peak main switch was only making the line conductor and there was no neutral if that could somehow create this fault, if there was a neutral to earth fault.

Or If the PEN conductor was open circuit on the supply how would that give the additional 12.5A ? Could it come from an adjacent property that also had the off peak heater on at the same time? I am assuming it must be another 3kW load as it seems too much of a coincidence to double the load. It's all made a bit harder due to the off peak supply not being on all the time.

I am summing that OMS suggestion regarding putting a screwdriver in the ground is assuming a broken PEN conductor or loss of neutral at one of the boards. If it were a broken neutral what would the expected result be?

Good fun innit
 25 July 2014 05:38 PM
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mapj1

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We know the cylinder is either connected to the supply MET or the mains water supply, (at least I think we do - you've reported R2 as OK).
OMS' suggestion is to check that there is not a large voltage between the nearer true ground of the plumbing and the suppliers neutral=PME CPC . It is possible for current to be coming up the water pipes and into your earthing circuit.

However this does not fit you reported 2 lots of 12.5A in phase & neutral adding to 25A - I'd have expected the resultant to be smaller, near zero unless something is happening that means the wire you have in your hand actually loops in a way that puts the neutral current thought the clamp part of the meter in the opposite direction - possibly some sort of shared neutral cock-up with the other heater - perhaps some current off the ring main. Is it practical to disconnect the other element completely including lifting the CPC. (13A plug or DP switch?) Right now you have effectively bonded the CPCs at the cylinder, which normally is OK, but we are in 'weird land' so simplifying would be good...
Where was that 25A measured actually- at the heating element or at the fuseboard end, and is it somewhere where the singles may be bundled in a way that makes it look like they come from one place, and not the other.

If there was a really odd fault where the supplied PME earth was interrupted or polarity reversed, so many other things would be wrong, I don't think it can be that. Mind you the immersion circuit could be L_N reversed. - normally the thermostat will open the live side when it gets hot - maybe it doesn't. Also, where the thermostats ON when passing the insulation tests?

Is it practical to swap the two elements wiring, so that they have some hot water, and see if the funny behaviour moves to the ring? Or put plug socket pairs in on flex and see what happens when its all off.
getting desperate I'm afraid - makes little sense so far - further experiment needed.

-------------------------
regards Mike
 25 July 2014 07:21 PM
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GeoffBlackwell

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Originally posted by: leckie

The building is a standard dwelling so I suppose the supplies are on the same phase. I haven't checked that but I would be amazed if they weren't . I ran out of time today as the off-peak tariff shut down as I was trying to sort out what was going on. I'm back there Saturday afternoon for another go.


Confirm that this is the case.

Regards

Geoff Blackwell
 25 July 2014 09:01 PM
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mapj1

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Hmm. Even if is the same phase the off peak part could have a wobbley neutral-earth issue though Id expect the meter contactor to be single pole I suppose it could be in the wrong one. - and are there storage heaters that also come on with it that would explain a 25A load if feeding backwards ? ? Or is it the kind with a diddly time switch in the meter, and an external contactor in the CU.
Does it work if you turn off the off-peak DB, and strap it into the normal CU instead. (thinking nothing more complex than a flex on a plug to the ring main.)

Also, did it work before, or are they new to the place?
and does the breaker always go?

As an alternate source of weird failures I've been caught once by arc-flash from a thermostat that only occurred when the load disconnected, and not always - perhaps needing contacts to open at the right phase (of the moon probably, given how hard it was to debug) The blue ball of plasma then presumably spread between the contacts and occasionally caught the earthed cover, ever so briefly shorting to ground.
Of course it never does with the covers off !. Fix was new stat, and surge suppression across it.
I'm not saying this applies here, but its on the Sherlock list - when all other avenues have been exhausted, whatever remains probably isn't it either. I don't think Conan Doyle actually did much fault finding, in general for me what remains are an infinite lists of more and more unlikely possibilities.

-------------------------
regards Mike

Edited: 25 July 2014 at 09:07 PM by mapj1
 25 July 2014 09:46 PM
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leckie

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Just back in so only just reading the replies. Many thanks for them.

I have drawn this out and can only get the 25 A bit with a series of faults. The job is very neat and you can clearly see what is going on in the consumer units. So no mixed up neutrals, etc, within the board. I am going back tomorrow to have a really good look. I will measure voltage between all relevant points and do the measurement to real earth.

I shall return tomorrow with all results. I cannot wait to see what is going on. But I suspect I may need some of you clever chaps to give me a little input to fully check out what's going on. Interestingly, the lady sent me a text today to say the mcb hadn't tripped again today. But I'm going back as I want to know where the 25A is coming from.

I have tested probably thousands of jobs ( I am very old). Never seen this before.

Edited: 26 July 2014 at 04:43 AM by leckie
 25 July 2014 09:46 PM
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UKPN

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still struggling?

i was wondering how oms and slittles "screwdriver in the park" idea is going? the topic seems to be going a bit slow? why not call the local DNO? always ready to solve those service problems!

Regards.
 25 July 2014 09:59 PM
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leckie

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Ah well, UKPN, I haven't been back there today, so I will have to wait until tomorrow to gather a little more info. It may not be a supply problem, DNO's are not always to blame, don't be do hard on yourself only joking !

But you are right, I am struggling to understand this problem, and I will most certainly be giving the DNO a call if it seems that it is a supply issue. You can PM me your mobile if you want and I will give you a call tomorrow night just as you are tucking into your Saturday night chicken madras
 25 July 2014 10:06 PM
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leckie

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Regarding two phases going to the property, I don't expect this to be the case deliberately. And if there were 400V present due to a fault, i.e. 120 degree shift coming back up the neutral, I would expect a few appliances to be exploding.

I'm still a bit unsure how to confirm a broken PEN.
 25 July 2014 10:23 PM
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UKPN

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forget the 2 phases, there will be 2 main fuses if there are, even then it could be 1 phase 3 wire.

you need to switch off all m/sws.
discon main earth from earth bar,
(no need to touch DNO service)

with your test lamp, between L and N on an open c/unit=half light.
as above, between L and an earth point =half light
as above between N and the earth point =no light
as above if between L and N no light, PME fault but thats not
going to happen.

if you get full light (very bright on your lamp you have 415 v so need to call us, leave every thing switched off.
 25 July 2014 10:35 PM
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leckie

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Thanks for that UKPN. My Martindale lamp will come out tomorrow!
 26 July 2014 04:44 PM
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leckie

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Hi all. I've been back today for additional testing to the circuits.
Checked the service arrangements. 100A TNCS single phase loops in and back out to another property. I meant to mention yesterday that I swapped the 16A mcb to the off peak Immersion heater and it hasn't tripped since, but it want to sort the other odd readings.

I first confirmed Zsb to both boards, Zs to both Immersion heater circuits, polarity, etc. All fine. There are two separate immersion heater circuits and I also measured the current on the line at the heater location and at the line and neutral individually at each consumer unit. All 12.5A. I measured around the line and neutral at the same time on the off peak circuit and it was still at 25A. I turned of the off peak imm heater mcb and there was no current on the line and 12.5A on the neutral. I put the mcb back on and I remeasured around the line and neutral and the 25A was gone? The 12.5A was now just present in the line and neutral. I assumed a load had go off, but the only things turned on in the house was the off peak Imm heater and a fridge was plugged in. I started to wonder if I had been measuring a neighbours immersion heater load that would have come on when the heat wise tariff switched on and had now switched off on the stat? No idea.

I tested voltages between L to N to E all combinations, all 240v. I tried to turn of the main switches to each board. Neither "clunked" open, they just pushed down. The output lines to each were off and I removed the neutral output links to both boards to the bus bars to avoid readings between the neutral and the earth link in the cut-out. The output of the neutral pole was still reading a short with the primary side to both switches? The live poles were both isolated. Now that's a bit of a coincidence!

I IR tested all circuit to both boards between L/N to E. The global readings for the off peak board was 33Mohms and 4.5Mohms for the on peak.

I reconnected everything, I didn't have replacement isolators so that will have to be done later. I also stuck a long screw driver in the ground and measured for volts between it and the earth bar of the consumer unit and got 0v.

Any more ideas on what's going on?
 26 July 2014 04:49 PM
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leckie

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I also checked Zs to both imm heater circuits, polarity, etc. Again, all fine.
 26 July 2014 04:56 PM
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slittle

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Originally posted by: UKPN

still struggling?



i was wondering how oms and slittles "screwdriver in the park" idea is going? the topic seems to be going a bit slow? why not call the local DNO? always ready to solve those service problems!



Regards.



Off topic, but a certain DNO has been looking for a fault for over a month and still can't find it so perhaps Ghostbusters might be a better option


Leckie,

The broken PEN trick is that a screwdriver in the grass gives you a reference to "real earth" as opposed to the PEN. If it was suspect and you measure between the MET and the screwdriver (I use a lump of earth rod but no matter) with the installation energised you would see volts and as load increased lots of volts. I'm fairly certain that's not what your problem is now you've done the other tests but still an odd one

Stu
 26 July 2014 05:14 PM
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andardry

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At this point I would seriously investigate the route of the off-peak cable, and ensure it doesn't disappear next door at some point, maybe even run a new cable from cu to immersion and see if problem vanishes, and watch out of window for plumber attending next door

-------------------------
Andrew
 26 July 2014 05:17 PM
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leckie

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Thanks Stu, yes I know it's referencing to real earth and that's what I was measuring, although I hadn't thought about the voltage if present rising with load. There was probably very little load when I tested for that. I might have to redo that one.

I just cannot think of a scenario that would give the 25A reading, and now with both the neutral poles on the isolators shorted out I am even more confused

However, it is now beer o'clock and I am off for a swift libation

Only one though as I have a dose of gout that is flipping sore!

I will return to this in an hour or two so I am hoping you clever clogs will be able to help.
 26 July 2014 05:19 PM
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leckie

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Good idea Andrew.

I will run a temp in for testing purposes.
 26 July 2014 05:26 PM
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mapj1

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Hm Do I understand the main switches in the DBs are single pole?
Never mind - that many megs of resistance eliminates a whole swath of stuff.

I measured around the line and neutral at the same time on the off peak circuit and it was still at 25A. I turned of the off peak imm heater mcb and there was no current on the line and 12.5A on the neutral.


Now that is a huge clue. Something else , not the immersion heater, is providing the 12,5A on that black wire. The $1meg question is what of course, if its supposed to be the only thing connected. I re-iterate my query about storage heaters.
No chance next door have drilled a hole and borrowed a power supply I suppose or there is an unexpected socket on that circuit? (joking aside I have seen, long ago when students like me had longer hair, a terrace house with no partition in the lofts, and what was clearly a power stealing arrangement that connected the two neighbouring lighting circuits. Un-believable, until seen and really badly done. Not sure what phase they were on, but it was back on their own soon after it was found. Never really got to bottom of it, as next door was rented too so could have been any previous occupant. Pre-dated loft insulation, so easily spotted..)

-------------------------
regards Mike

Edited: 26 July 2014 at 09:10 PM by mapj1
 26 July 2014 09:27 PM
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leckie

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No the main switches are not single pole. They are double pole MK isolators for consumer units. However, both of the neutral poles have gone short circuit! Now I think that is strange. I have disconnected the outputs of the neutral pole so no short via a pme link, etc. so what would cause BOTH of the neutral poles to become short circuit? Very strange I think.

There is one storage heaters but it off on the controls as it is a little warm at the moment. However I do now recall that the double pole switch supplying it is switched on. But how would that give 12.5A on L and N individually and 25A across both? The lives are in separate MCB's?

Also, say somehow the supply from the off peak Imm heater supply went into next door and was connected to another 3 kW load, the line and neutral would measure about 25A individually and about zero if clamped around L and N together wouldn't they?
 26 July 2014 09:33 PM
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mapj1

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Ah well only if next door had done their hacking right, If they are on another phase, and manage to return current coming through their live via your neutral, maybe not as it would part cancel instead of add.

But that's really not a likely situation.

Now, those shorted neutrals - are the isolators really faulty and not opening their neutral contacts, or would the same symptoms exist if the switches were OK but neutrals of one of the off -peak and a normal one met up somewhere they should not. like a water heater time switch no one has found yet down the back of the airing cupboard?

-------------------------
regards Mike
 26 July 2014 09:45 PM
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leckie

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The isolators:
The input neutrals are connected into the top and there is nothing connected into the bottom of the neutral pile, completely empty, and they read short circuit on that pole but not on the line pole. Also the switches do no clunk open or shut. They just press down. I tried three isolators in my stores earlier. They all went "clunk". I.e. A very positive switching action. The existing isolators a knackered, had it, passed away, deceased! But they are stuck closed on the neutral connection so power is still available. There is no sign of thermal damage to the isolators. I would imagine the only thing that would bugger up the isolators would be excess current. Unless they were part of a bad batch, but I simply do not think that is likely.
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